Application of regionalized multiplet stripping to retrieval of aspherical structure constraints

Abstract

A new technique for the analysis of low-frequency seismic recordings is introduced with which constraints on the Earth's large-scale aspherical structure can be obtained. The technique, which we call 'regionalized multiplet stripping', applies multiplet stripping (Gilbert & Dziewonski, Phil. Trans. R. Soc. Lond., A, 278, 187-296, 1975) to sets of seismograms for which sources and receivers share a common great circle. Multiplet stripping applied to such subsets reveals not only the well-known frequency shifts of fundamental modes but also similar frequency shifts for a large number of overtones. Geographical patterns of overtone frequency shifts can subsequently be related to aspherical structure in an identical manner to the fundamental modes. The newly estimated structure coefficients of overtones provide improved depth resolution for the lowest-order aspherical structure. So far we have retrieved degree s = 2 aspherical structure coefficients for 271 spheroidal overtones with harmonic degree f > 10. In addition, structure coefficients for s = 4 could be retrieved for a few well-excited low-n branches. For the fundamental modes, we find very good agreement with single-record peak frequency analyses. Predictions from recent 3-D mantle models generally agree well with our new structure coefficients.